L. Daza

Optimisation of flow-field in polymer electrolyte membrane fuel cells using computational fluid dynamics techniques

Abstract The purpose of this work was the enhancement of performance of Polymer Electrolyte Membrane Fuel Cells (PEMFC) by optimising the gas flow distribution system. To achieve this, 3D numerical simulations of the gas flow in the assembly, consisting of the fuel side of the bipolar plate and the anode, were performed using a commercial Computational Fluid Dynamics (CFD) software, the “FLUENT” package. Two types of flow distributors were investigated: a grooved plate with parallel channels of the type commonly used in commercial fuel cells, and a porous material. The simulation showed that the permeability of the gas flow distributor is a key parameter affecting the consumption of reactant gas in the electrodes. Fuel utilisation increased when decreasing the permeability of the flow distributor. In particular, fuel consumption increased significantly when the permeability of the porous material decreased to values below that of the anode. This effect was not observed in the grooved plate, which permeability was higher than that of the anode. Even though the permeability of the grooved plate can be diminished by reducing the width of the channels, values lower than 1 mm are difficult to attain in practice. The simulation shows that porous materials are more advantageous than grooved plates in terms of reactant gas utilisation.

Development and performance characterisation of new electrocatalysts for PEMFC

New electrocatalysts based on Pt, Pt-Ru and Pt-Pd have been prepared by the microemulsion method. This method allows the production of a very narrow size distribution of metal particles, with an av ...

In situ high temperature neutron powder diffraction study of oxygen-rich La2NiO4+δ in air: correlation with the electrical behaviour

The knowledge of the thermal evolution of the crystal structure of a cathode material across the usual working conditions in solid oxide fuel cells is essential to understand not only its transport properties but also its chemical and mechanical stability in the working environment. In this regard, high resolution neutron powder diffraction (NPD) measurements have been performed in air from 25 to 700 °C on O2-treated (350 °C, 200 bar) La2NiO4+δ. A structural transition from the orthorhombic Fmmm to the tetragonal F4/mmm space group takes place at about 150 °C. The reversibility of this transition has been determined to be strongly dependent on the sample oxygen content. The structural data have been correlated with the transport properties of this layered perovskite. The electrical conductivity of O2-treated La2NiO4+δ exhibits a dirty-metal (high T)-to-semiconducting (low T) transition as a function of temperature, displaying a maximum value of 82 S cm−1 at around 400 °C. The largest conductivity corresponds, microscopically, to the shortest axial Ni–O2 distance (2.19(1) A), revealing a major anisotropic component for the electronic transport. The interstitial oxygens occupy the 16j and 16e positions in the low and high temperature phases, respectively. The refined oxygen occupancy from NPD data is in quite good agreement with the thermogravimetric data. Good thermal stability of the oxygen content has been observed in the studied temperature range, as required for practical applications.

Effect of Sr content on the crystal structure and electrical properties of the system La2−xSrxNiO4+δ (0 ≤x≤ 1)

Materials formulated as La2−xSrxNiO4+δ (0 ≤ x ≤ 1) have been prepared and investigated by high-resolution neutron powder diffraction in order to correlate the structural variation induced by the incorporation of Sr into the crystal lattice with the electronic and thermal properties of each material. The evolution of the electrical conductivity and thermal expansion coefficients with temperature have been determined in order to study the potential use of these compounds as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFC). These oxides show a good thermal expansion coefficient (TEC = 11–13 × 10−6 K−1), and high electronic conductivity up to 273 S cm−1. It is noticeable that a great enhancement of the electrical conductivity with the Sr content is concomitant with the shortening of the Ni–O1 bond length.

Electrochemical behaviour of lithium–nickel oxides in molten carbonate

Impedance spectroscopy was used to investigate the stability and the catalytic activity of the lithium–nickel mixed oxides with high lithium content (LixNi1� xO, x ¼ 0:30–0.40) in a eutectic melt (Li:K) at 650 8C under a corrosive atmosphere (CO2:O2 4:1). The results were compared with a NiO reference cathode material. A modified transmission line model allowed to give a physical-meaning to the impedance spectra. All Li–Ni oxides showed similar catalytic activity and their impedance values were one order of magnitude lower than NiO. During the first 100 h of immersion, the samples showed structural changes due to the loss of lithium. Later on, the structure kept stable. The loss of lithium was confirmed by chemical analysis and X-ray diffraction (XRD). All Li–Ni oxide samples reduced the nickel dissolution in the eutectic in one order of magnitude in relation to NiO. In general, similar morphology was observed by scanning electron microscopy (SEM) for the fresh samples. After their immersion, the Li–Ni oxides did not show morphological change except for the sample with x ¼ 0:30, for which a reduction of the particle size was observed. NiO presented an important morphological change due to its lithiation in situ. # 2003 Elsevier Science B.V. All rights reserved.

Study of a Li-Ni oxide mixture as a novel cathode for molten carbonate fuel cells by electrochemical impedance spectroscopy

Li–Ni oxide mixtures with high lithium content are considered to be an alternative cathode material for molten carbonate fuel cells (MCFCs). The electrochemical behaviour of Li0.4Ni0.6O samples has been investigated in a Li–K carbonate melt at 650 °C by electrochemical impedance spectroscopy as a function of immersion time and O2 and CO2 partial pressure. The impedance spectra have been interpreted using a transmission line model that includes contact impedance between reactive particles. The Li0.4Ni0.6O powder particles show structural changes due to high lithium leakage and low nickel dissolution from the reactive surface to the electrolyte during the first 100 h of immersion. After this time, the structure seems to be stable. The partial pressures of O2 and CO2 affect the processes of oxygen reduction and Li–Ni oxide oxidation. X-ray diffraction and chemical analysis performed on samples before and after the electrochemical tests have confirmed that the lithium content decreases. SEM observations reveal a reduction in grain size after the electrochemical tests.